On the Dissolution of Platinum: From Fundamental to Advanced Catalytic Materials

Abstract

The polymer electrolyte membrane fuel cell (PEMFC) is a promising electrochemical energy conversion device for use in electric vehicles (EVs). Several auto manufacturers have begun production; however, the cost and lifetime of the PEMFC remain a major roadblock to widespread market penetration. Significant cost comes from the use of nanoparticulate platinum dispersed on carbon (Pt/C) as an electrocatalyst in the catalyst layer (CL), necessary to overcome the sluggish kinetics of the oxygen reduction reaction (ORR). To reduce the required quantities of platinum, the catalytic activity towards the ORR must be improved in order to meet performance requirements. Therefore, a great deal of research has been aimed at improving activity towards the ORR. Many strategies now exist, such as shape-controlled nanoparticles, particle size effects, alloying with transition metals and incorporation of ionic liquids with the solid catalyst ionic liquid layer technique (SCILL). The lifetime of PEMFCs is hindered by the degradation of the CL, in which the two main primary degradation mechanisms have been identified as platinum dissolution and carbon corrosion. Carbon corrosion may be avoided through the use of alternative supports, such as corrosion resistant metal oxides, which have also become a popular topic of research. The impact on degradation of the above mentioned strategies remains larger unexplored. Specifically, Pt dissolution at low loadings, different particle sizes, with transition metal alloying, ionic liquids (SCILL) and with metal oxide supports present a wide variety of variables to consider on the road to reduced cost and increased lifetime of PEMFCs. Therefore, such systems were studied systematically from fundamental aspects, to state-of-the-art materials to advanced new catalysts. A systematic approach was used to study the dissolution of platinum, platinum alloys and platinum on metal oxide supports in order to address their stability. First the dissolution of low index Pt facets was studied and facet dependent results emerged, which may be hypothesized to dictate the stability of shape-controlled nanoparticles. A large impact of Pt nanoparticle density (wt. % Pt on carbon) on dissolution was revealed, with low loadings significantly increasing Pt dissolution, suggesting future CL designs must balance reduced loadings with stability concerns. Furthermore, Pt density is directly related to interparticle distance, which drastically impacts the redox behaviour of Pt and must be considered by researchers in future investigations on degradation. Varying particle size showed increased stability for larger particles, indicating a necessary balance between Pt utilization and degradation in CL optimization. Moving to advanced new ORR catalytic materials, using the SCILL on Pt/C decreased Pt dissolution and increased ORR activity, attributed to selective ionic liquid binding on low-coordinated sites, revealing an impressive result. However, advanced experimental materials may introduce unexpected degradation mechanisms. When the SCILL approach was employed for a trimetallic PtNiMo/C alloy system, incorporation of the ionic liquid (IL) caused enhanced dissolution and decreased stability. Investigation of metal oxides supports revealed that not all offer increased stability compared to carbon. For Pt on indium tin oxide, high rates of support dissolution were observed, in comparison to the relatively stable ruthenium titanium oxide. Surprisingly, a unique mode of degradation was uncovered, in which the active Pt nanoparticles become partially encapsulated by the support. These investigations expose key aspects of the dissolution stability of ORR catalytic materials that may aid in the design of future devices as well as new catalytic materials. Show more